It allows avoiding allocations, but it doesn't allow using serialised data as a backing memory for an in-language type. Protobuf varints have to be decoded and written out somewhere. They cannot be lazily decoded efficiently either: order of fields in the serialised message is unspecified, hence it either need to iterate message over and over finding one on demand or build a map of offsets, which negates any wins zero-copy strives to achieve.

Those field accessors take and return string_view but they still copy. The official C++ library always owns the data internally and never aliases except in one niche use case: the field type is Cord, the input is large and meets some other criteria, and the caller had used kParseWithAliasing, which is undocumented.

To a very close approximation you can say that the official protobuf C++ library always copies and owns strings.